The present invention pertains to cellular telecommunications, and particularly to compressed mode (e.g., also known as slotted mode) control in a WCDMA mobile radio system.
In mobile telecommunications, a mobile station (MS) such as a mobile telephone communicates over radio channels with base stations. Each base station usually transmits and receives signals over selected radio channels for a particular geographic region known as a cell. The cell often is subdivided into several sectors. Typically a plurality of base stations are connected to a base station controller node, also known as an exchange or a radio network controller node (RNC). One or more RNCs are, in turn, connected to or included with a mobile switching center (MSC). The mobile switching center is usually connected, e.g., via a gateway, to other telecommunication networks, such as the public switched telephone network or to a packet-data network such as the Internet.
In a code division multiple access (CDMA) mobile telecommunications system, the information transmitted between a base station and a particular mobile station is modulated by a mathematical code (such as spreading code) to distinguish it from information for other mobile stations which are utilizing the same radio frequency. Thus, in CDMA each mobile radio employs its own unique code sequence to encode its data signal. The receiver, knowing the code sequences of the mobile radio it services, decodes the received signal to recover data from each radio.
The CDMA encoding process enlarges the spectrum of the signal and is therefore known as spread-spectrum modulation. The resulting signal is also called a spread-spectrum signal, and CDMA is often denoted as spread-spectrum multiple access. The spectral spreading of the transmitted signal gives to CDMA its multiple access capability. That is, if multiple radios transmit a spread-spectrum signal at the same time, the receiving station will still be able to distinguish between the radios because each user has a unique code that has a sufficiently low cross-correlation with other codes used simultaneously by other radios.
Correlating the received signal with a code signal from a certain radio despreads the signal from that radio, while the other spread-spectrum signals will remain spread over a large bandwidth. Thus, after decoding a signal from a particular radio within the information bandwidth, the power of the desired radio signal will be larger than the interfering power of the other radios. With that power discrepancy, the desired signal can be extracted.
In a CDMA system, power control is very important. In the uplink direction, the requirement for power control arises because of the possibility for multiple access interference. All radios in a cell using a CDMA system transmit their data using the same bandwidth at the same time as other radios in that cell. Further, in a CDMA system the neighboring cell frequencies are the same as in a given cell. So interference can be seen into neighboring cells, causing capacity degradation. In such a system, it is inevitable that radios will interfere with one another. Signals received by the base station from a radio close to the base stations, for example, will be stronger than signals received from radios located at a cell boundary. Distant radios will thus tend to be dominated by close ones. To maintain capacity, all signals, regardless of distance, arrive at the base station with the same mean power by controlling the radios to achieve a constant received mean power for each user.
In contrast to the uplink, in the downlink, all signals from a base station propagate through the same channel and thus are received by a mobile station with equal power. Power control on the downlink is not required to eliminate the near-far problem, but is required to minimize or offset interference with neighboring cells.
These power controls require certain measurements of signal strength, signal loss characteristics, etc. to be taken. In some cases, the mobile radio participates in obtaining those measurements.
The mobile radio measurements are also be used to evaluate soft, softer and hard handovers. In soft handover a mobile station is connected to more than one base station simultaneously. Softer handover is a soft handover between two sectors of a cell. A mobile station performs a hard handover when the signal strength of a neighboring cell exceeds the signal strength of the current cell within a given threshold.
In order to avoid the various forms of interference, instantaneous handovers occur between cells, sectors, and base stations as needed when the signal strength after the change would exceed the signal strength of the current conditions and to allow the mobile station to connect into a cell from which it receives a signal with the highest power (i.e., with the lowest path loss). For example, a mobile station may enter a soft handover when the signal strength of neighboring cell exceeds a certain threshold but is still below the current base station""s signal strength.
Further, mobile station comparative measurements are taken to optimize a mobile radio""s transmissions when multiple frequency carriers exist in a cell. This exists, as an example, in hierarchical cell structures where micro cells will have a different frequency than the macro cell overlaying the micro cell. For those inter-frequency handovers, the mobile station has to be able to measure the signal strength and quality of another carrier frequency, while still maintaining the connection in the current carrier frequency. Since a CDMA transmission is continuous, there are ordinarily no idle slots for the inter-frequency measurement to occur.
Two proposals are presented to address the mobile radio""s need to measure on one channel while continuously receiving on another channel, namely: 1) the so-called compressed mode and 2) the use of a dual receiver mode. In the compressed mode, measurement slots are created by transmitting the data of a frame, for example, with a lower spreading ratio during a shorter period, and the rest of the frame is utilized for the measurements on other carrier frequencies. In the second alternative, mobile stations are equipped with dual receive mode capability so they can receive on two channels simultaneously. The dual receiver can measure other frequencies without affecting the reception of the current frequency.
In CDMA mobile communications, typically the same base band signal with suitable spreading is sent from several base stations with overlapping coverage. The mobile terminal can thus receive and use signals from several base stations simultaneously. Moreover, since the radio environment changes rapidly, a mobile station likely has radio channels to several base stations at the same moment, e.g., so that the mobile station can select the best channel and, if necessary, use signals directed to the mobile from various base stations in order to keep radio interference low and capacity high.
One weakness of conventional cellular telecommunications networks is narrowband radio access. Newer radio access systems provide wireless access at very high data rates and support enhanced bearer services not realistically attainable with earlier generation mobile communication systems. One such system is the Wideband-Code Division Multiple Access (W-CDMA) radio access network. Unlike narrowband access methods such as Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA), and to some extent xe2x80x9cregularxe2x80x9d CDMA, W-CDMA supports greater bandwidths and improves the quality of service by providing robust operation in fading environments and transparent handoffs between base stations.
A universal mobile telecommunications system (UMTS) that provides communications with mobile radios, including multimedia communications, utilizing a UMTS Radio Access Network (URAN), is disclosed in U.S. Provisional Patent Application Serial No. 60/080,548 for xe2x80x9cRadio Access in Universal Mobile Telephone System, filed Apr. 3, 1998, which is incorporated herein by reference. The URAN includes plural base stations for communicating with mobile radios over a radio air interface using radio channel resources allocated by a radio network controller connected to the base stations. First and second external network service nodes that interface with first and second external networks, respectively, communicate with the URAN over a radio access network interface. The URAN provides a radio access bearer service to the service nodes. When one of the service nodes requires communication with a mobile radio, the service node requests a radio access bearer from the URAN rather than a specific radio channel resource. A radio access bearer is a logical channel through the URAN and over the radio air interface corresponding to a single data stream. For example, one bearer xe2x80x9ccarriesxe2x80x9d a speech connection, another bearer carries a video connection, and a third bearer carries a data packet connection. Bearers are logical channels mapped onto physical channels. The radio access bearers are dynamically assigned to radio channel resources solely by the URAN. The radio access bearer service and the radio access network (RAN) interface isolate the details of radio resource handling, radio channel allocation, and radio control, e.g., soft handoff, which are managed by the URAN from services requested and managed by the service nodes, e.g., call control and supplementary service requests, authentication, mobility management, etc.
In WCDMA, interfrequency handovers used in hierarchical, macro, and micro cells require the taking of measurements on other frequencies while maintaining a continuous connection on a current frequency. Such handovers can also occur between a WCDMA system and a second-generation system, like GSM or IS-95. In order to complete interfrequency handovers, measurements on other frequencies than the current frequency can be taken care of by either the dual receiver mode or compressed mode proposals.
That is, if the mobile radio has dual receiver mode capability, during the interfrequency measurements, one receiver branch is switched to another frequency for measurements, while the other keeps receiving the WCDMA data stream from the current frequency. The advantage of the dual receiver mode approach is that there is no break in the current frequency connection.
In the compressed mode approach, the mobile station is given spare time to measure other WCDMA frequencies (or a second generation frequency). Ordinarily, compressed mode exists only on the downlink or simultaneously on the downlink and uplink (if the current transmission disturbs the measurement taken on another frequency, for example, measuring PCS1900 when in WCDMA mode).
The current state of the art has the compressed mode operation being controlled by the network (UTRAN). Under that system, when the UTRAN requests a mobile to deliver measurements on another frequency, it also defines the slot and defines when the mobile can enter compressed mode. This can be done as defining a periodic compressed mode, e.g., every N frames starting frame M, or a single compressed mode frame, e.g., frame M.
The current system has problematic consequences associated with the fact that a network can expect to encounter different mobile station implementations, each of which may require different amounts of compressed mode to perform inter-frequency measurements. Further, the network must recognize when a particular mobile station has a dual receiver mode, since, in that case, no slot is required at allxe2x80x94but that is pre-conditioned on the mobile station at issue being equipped with an extra receiver. The specific and varied mobile station types thus remains at issue in deciding whether compressed mode is necessary; and if so, how much slot is necessary. Further, some mobile stations may require compressed mode both on uplink and downlink, depending on what is to be measured. Others will require it only on downlink.
The requirements for compressed mode will also differ depending on what is to be measured by the mobile station. The slot may be different when the mobile station is measuring another WCDMA frequency, another WCDMA frequency in another (future defined) band, a TDD frequency in the UMTS band, a PCS1900 frequency, a DCS 1800 frequency, a GSM 900 frequency, a PDC 1500 frequency, etc. Thus, there will be many different types of dual receiver mode implementations of (WCDMA+another system).
When the network controls the compressed mode for these terminals, it has to either order compressed mode unnecessarily often (for terminals not requiring frequent compressed mode), or it has to take the different terminal capabilities into account. As seen from the examples above, the description of the capabilities of the terminals may become very complex, such that the information needed by the network to control the compressed mode is inefficiently large.
In the present solution to the above problems, the compressed mode is controlled by the mobile. In an example implementation, the mobile will make a request to the UTRAN to enter compressed mode (or change an existing compressed mode scheme) in a particular frame and the UTRAN will respond to acknowledge this. The specifics for entering compressed mode may still be defined as a single-frame compressed mode or as a periodic compressed mode scheme.
These, as well as other objects and advantages of this invention, will be more completely understood and appreciated by careful study of the following more detailed description of a presently preferred exemplary embodiment of the invention taken in conjunction with the accompanying drawings, of which: